Familiar defendant admits to murder, initially (10)
I believe the answer is:
accustomed
'familiar' is the definition.
(I've seen this before)
'defendant admits to murder initially' is the wordplay.
'defendant' becomes 'accused' (synonyms).
'admits' is an insertion indicator (to admit is to allow in).
'initially' indicates taking the first letters.
The first letter of 'murder' is 'm'.
'to'+'m'='tom'
'accused' going around 'tom' is 'ACCUSTOMED'.
(Other definitions for accustomed that I've seen before include "familiar" , "Used to" , "usual" .)